This invention relates to semiconductor devices that include a silicon oxide/silicon nitride/silicon oxide (ONO) structure, and to methods for oxide formation in such devices.
In a conventional process for making a device having an ONO structure, a tunnel layer, a silicon nitride layer and a top oxide layer are formed over a substrate, and then an etching process is performed for patterning the ONO structure.
The top oxide layer can be formed by oxidation of the silicon nitride. However, in conventional processes oxidation of silicon nitride is time-consuming and has a very high thermal budget. In some conventional processes, for example, silicon nitride is oxidized by wet oxidation within a furnace at a temperature of 1000° C. over a time period as long as 60 minutes.
Moreover, the oxidation step in such a conventional process is followed by a cleaning process, which may etch the top oxide layer, even to the extent of exposing the corners of the silicon nitride layer so as to cause a leak from the silicon nitride layer to a polysilicon gate formed subsequently. As a result, the charges stored in the silicon nitride layer will be lost, causing an electrical defect in the device.
In one conventional approach to avoiding loss of the top oxide layer during the cleaning process, a tunnel oxide layer and a silicon nitride layer are first deposited and patterned, and then a top oxide layer is grown on the silicon nitride by wet oxidation. However, the oxidation selectivity of wet oxidation for the substrate and the silicon nitride layer is relatively high, that is, the oxidation rate of wet oxidation for the substrate is far greater than that of the silicon nitride layer. Using a wet oxidation in this manner to form a 100 Å thick top oxide layer, for example, will result in a 1000 Å thick oxide layer formed on the substrate. Consequently, where a conventional wet oxidation is used, it is necessary to remove the thicker oxide layer formed on the substrate. This makes for a more complicated process, and can result in an uneven substrate surface.
One approach to growing an oxide on a substrate is referred to as the in situ steam generation (“ISSG”) process. In the “ISSG” process, the substrate (typically a semiconductor wafer) is heated to a temperature high enough to catalyze a reaction between an oxygen-containing gas and a hydrogen-containing gas to form oxygen radical. Then the reactive oxygen radical can effectively oxidize the silicon or silicon nitride on the substrate.
U.S. Pat. No. 6,184,155 describes a two step ISSG process for growing an ultra-thin silicon dioxide gate insulator layer for narrow channel length MOSFET devices. A first steam oxidation and in situ anneal in a nitrous oxide ambient is followed by a second steam oxidation and in situ anneal in a nitrous oxide ambient. The two-step procedure is said to result in a silicon dioxide layer having thickness between about 10 Δ and 20 Δ, providing a gate insulator having reduced leakage current during standby or operating modes, as compared with silicon dioxide gate insulators formed by procedures not employing two-step ISSG.
U.S. Pat. No. 6,171,911 describes a process for selectively and sequentially forming two different thicknesses of thermally grown silicon oxide in a MOSFET device. Regions on the wafer are defined by field isolation. Conventional oxidation methods for making a gate oxide are used to grow a thicker oxide on all the exposed regions; then the wafer is masked and the mask is patterned to expose regions that are to receive a thinner oxide, and the thicker oxide is removed from those regions by wet etching using hydrofluoric acid; then the mask is stripped and the wafer is cleaned with an aqueous solution free of hydrofluoric acid. Then the wafer is subjected to a low pressure rapid thermal anneal (600° C. to 1050° C.) in an ambient containing hydrogen and nitrogen to remove native oxide and to passivate the silicon surface, reducing the residual oxide thickness to about 4 Δ. This is said to result in improved thickness uniformity and oxide quality, and the residual oxide film following annealing is said to become a more robust form of silicon oxide.